Well done! I'm playing with it now, this is functional, elegant, usable, and I see you paid attention to a lot of smaller usability details already (warning before closing unsaved file, etc..). Very impressive job you did!

The icon that is looked for by the addon manager is ADDONNAME_workbench_icon.svg where ADDONNAME is the addon name as it appears in the Addon manager and on https://github.com/FreeCAD/FreeCAD-addons in your case BCFPlugin

Ok thanks! I renamed the icon now, while leaving it still in the icons directory. Does it get picked up automatically from there by the addon manager though?